The invention relates to a method of making a high-pressure accumulator pipe of steel for fuel injection systems as well as to a high-pressure accumulator pipe.
High-pressure accumulator pipes for fuel injection systems of internal combustion engines are also designated as common rail system. Pressure generation and fuel injection are decoupled from one another in the common rail system.
A separate high-pressure pump produces continuously pressure which is stored in the central high-pressure accumulator independently from the injection sequence. High-pressure lines extend from this accumulator to the individual injectors which are associated to the motor cylinders. The built-up pressure is constantly available in the fuel line.
in order to satisfy the high demands on the mechanical properties and the corrosion and cavitation resistances, when the injection pressures of today reach up to 1800 bar, components which assume to a lesser degree a storage function but rather a conducting function, such as the high-pressure fuel lines, e.g. known from the documents DE 203 17 565 U1, DE 198 08 012 C2, and DE 197 16 659 C2, are designed as composite pipes.
These composite pipes include a relatively thin-walled inner pipe part and an relatively thick-walled outer pipe part which are connected to one another via a press-fit. The press-fit is realized for example through cold drawing (DE 197 16 659 C2).
The inner pipe part is made of a high-alloy, corrosion-resistant and high-strength steel as a consequence of the direct contact with the fuel mixture under high pressure, while the outer pipe part is typically made of unalloyed or low-alloy steel.
High-pressure accumulator pipes which do not assume a conducting function but predominantly a storage function, must exhibit a high fatigue strength besides the required mechanical properties in order to be able to withstand the high and pulsating pressures during operation. This requires the presence of a superior inner pipe surface with a very low roughness that cannot be realized using known production methods for high-pressure accumulator pipes. For example, roughness values of Rz≦1.0 μm and Ra≦0.2 μm should be reached.
High-pressure accumulator pipes are disclosed e.g. in DE 10 2004 030 394 B3 and DE 199 36 685 A1.
In order to satisfy the high surface demands, it is known to make the high-pressure accumulator pipes from a pipe comprised only of one pipe part through deep drilling of solid material or to use cold-finished, seamless or welded precision steel pipes which are cold-rolled in two draws.
Deep drilling has shortcomings relating to the high material consumption and the complicated deep drilling process. The surface quality and the properties of the marginal drilling zone oftentimes do not meet the demands and the required high fatigue strength can only be realized through an additional autofrettage process.
As cold-finished pipes require two drawing processes, high costs are incurred and the pipes oftentimes exhibit insufficient surface quality and inadequate properties at the marginal zones, and the application of a cost-intensive autofrettage is also necessary. DE 103 03 853 A1 discloses a high-pressure accumulator pipe which involves a composite pipe of two pipe parts which are connected to one another through heat application and shrinkage. This is also disadvantageous because the inner pipe part produced by deep drilling.